In commercially-available thermal ink-jet color printers, such as the DeskJet.RTM. printer available from Hewlett-Packard Company, a color spectrum is achieved by combining cyan, magenta, and yellow inks in various proportions. The cyan, magenta, and yellow inks derive their hues from cyan, magenta, and yellow dyes, respectively. The particular set of dyes so employed constitutes a so-called "dye set". Color printers typically employ a four-pen set containing cyan, magenta, and yellow inks as well as a black ink.
Thermal ink-jet inks are commonly available as aqueous-based ink compositions that are formulated by dissolving dye in an ink vehicle. For example, a cyan ink would comprise a cyan dye dissolved in an ink vehicle. The dye molecules employed in ink-jet ink compositions are often in the form of dye salts made of a dye anion and a cation such as sodium. These dyes are designed to form solids in the target paper substrate by way of crystallization of the dye salt.
Controlling the behavior of printed ink compositions before crystallization of the dye salt is crucial in attaining good print quality. For example, many thermal inkjet inks, when printed in various colors on paper substrates, tend to bleed into one another. Bleed occurs as colors mix both on the surface of the paper substrate as well as within the substrate itself. It is desirable to have a border between colors that is clean and free from the invasion of one color into the other. One solution to the problem of color to color bleed involves increasing the penetration rate of the ink into the paper with the use of surfactants. However, increasing the penetration rate in this fashion may also reduce edge acuity. A solution to the specific problem of black to yellow color bleed is disclosed in U.S. Pat. No. 5,198,023, issued Mar. 30, 1993 and assigned to the same assignee as the present application, wherein multivalent cations such as calcium chloride and magnesium chloride are added at concentrations ranging from about 1 to 10 wt. % to yellow cationic inks to prevent bleed between yellow and black inks.
Reducing the bleed observed for a single ink is a somewhat piecemeal approach in that it fails to optimize print quality for a four-pen set. To achieve good overall print quality from a color printer such as the DeskJet.RTM. printer, a holistic approach is needed in which the cyan, magenta, and yellow inks are each specifically formulated to achieve reduced color to color and black to color bleed.